INTRODUCTION

Combustion of hydrocarbon fuels is the most used means of power production in the United States. However, its current application is mostly limited to the macroscale, while electrochemical sources are primarily used for power generation on the microscale. Since liquid hydrocarbon fuels have energy densities on the order of 50 times greater than electrochemical sources, combustion of hydrocarbon fuels on the microscale appears to be an attractive alternative to electrochemical sources of power production. Microscale combustion technology would produce lighter and longer lasting power sources for microdevices and remove design constraints stemming from the use of electrochemical means of power production from microdevices. Research into the use of hydrocarbon fuels on the microscale is being undertaken in the hope that the advantages derived from combustion sources can be utilized to improve microscale power generation. Though many different approaches are being taken to achieve microscale hydrocarbon combustion power generation, all share the common goal of producing electrical energy from the combustion of hydrocarbon fuel.

Micro Gas Turbine Engines

One approach to microscale hydrocarbon combustion power generation sources is the development of a miniature gas turbine. J. Peirs, D. Reynaerts, and F. Verplaetsen of Katholieke Universiteit Leuven [1] have tested a stainless steel single-stage axial impulse turbine with a rotor diameter of 10mm. This turbine was tested with compressed air at speeds of up to 160,000rpm, and was capable of generating a mechanical output of 28W with 18.4% efficiency. To generate an electrical output, the turbine was connected to a small electric generator, and produced a maximum electrical power of 16W, with a total system efficiency of 10.5%. Using these results, the electrical power density of the microturbine was found to be 240W/kg by dividing the electrical power produced by the mass of the turbine and generator. The design of the turbine was then improved to help eliminate losses, and tested with the generator. The improved turbine produced a maximum electrical power of 44W, with a minimum of 30W, depending on the supplied air temperature. Future work will include testing the turbine with a compressor in a complete Brayton cycle, with the addition of a combustion chamber to reach higher turbine speeds.

C.M. Spadaccini, J. Lee, S. Lukachko, and I.A. Waitz of Massachusetts Institute of Technology along with A. Mehra of D-STAR Engineering Corporation and X. Zhang of BostonUniversity [2] tested two microcombustors designed to power a microscale gas turbine engine. The first was a six-wafer design in which either air or premixed fuel-air enters axially and makes a 90 degree turn before entering a compressor. Following the compressor, the air or fuel-air flows through a cooling duct that wraps around the combustion chamber. If the fluid in the duct is air, fuel is injected into the duct. The mixture is then burned in the combustion chamber. From the combustor, the exhaust would then be passed through a turbine in the full engine. The second was a dual-zone microcombustor, which modified the six-wafer design by connecting the cooling duct to the combustion chamber via a series of holes, allowing inlet air to bleed into the combustion chamber. This air splits the chamber into two zones and cools the combustion products to 1600K, which is the desired turbine inlet temperature.

The six-wafer microcombustor was tested with premixed hydrogen-air at varying mass flow rates and equivalence ratios. At a mass flow rate of 0.11g/s and a pressure of 3atm, exit temperatures exceeded 1600K and efficiency exceeded 90%, and power density was approximately 1100MW/m3. The dual-zone microcombustor achieved exit temperatures above 1600K when the mass flow rate exceeded 12g/s and efficiency exceeded 85%. Compared to the six-wafer design, exit temperatures were lower and the operating range was much widerin the dual-zone microcombustor. When the microcombustors were tested with hydrocarbon fuels, exit temperature, efficiency, and power density dropped for both designs. For ethylene-air, power density was approximately 500MW/m3 for the six-wafer and 100MW/m3 for the dual-zone. For propane-air, power density was approximately 140MW/m3 for the six-wafer and 75MW/m3 for the dual-zone. Also, hydrocarbon fuels reduced the mass flow rate range for the dual-zone microcombustor due to the short mixing length but increased the equivalence ratio range.

Microscale Catalytic Combustion

In spite of the advantages to be derived from the use of combustion systems for power generation, they have not been used on the microscale due to difficulties encountered when reducing combustion systems to the microscale. Catalytic combustion is being explored by many as a way to overcome these difficulties. R.B. Peterson of Oregon State University and J.A. Vanderhoff of the Army Research Laboratory Aberdeen Proving Ground [3] identify heat loss and wall quenching of free radicals as the largest obstacles to reducing combustion systems to the microscale. To attempt to reduce the effects of radical wall quenching, a device was built that sought to use a catalyst to cause heterogenous reactions. The device burned an H2-air mixture at the end of a counterflow heat exchanger in a vacuum chamber, where the heat exchanger was used to heat the reactants to temperatures that allow catalytic combustion to occur. The vacuum chamber and the lack of contact with any material surfaces of the hot end of the heat exchanger left radiation as the primary source of heat loss. The catalyst was a 0.025mm platinum wire. When the experiment was run at varying mass flow rates, the temperature of the hot end of the heat exchanger ranged from 650K to 1280K, well below the calculated adiabatic flame temperature of 2161K. The total energy loss from all sources in the combustor was 650mW at a mass flow rate of 6.09microgram/s.

J. Vican, B.F. Gajdeczko, F.L. Dryer, D.L. Milius, and I.A. Aksay of PrincetonUniversity and R.A. Yetter of the PennsylvaniaStateUniversity [4] developed a “Swiss roll” alumina ceramic microreactor with platinum coated internal walls. The “Swiss roll” design utilizes a spiral channel shape to preheat the reactants and to allow maximum heat transfer through the walls and to external devices. The catalyst lowers the temperature required for ignition of the hydrogen-air mixture used to test the device. The device was tested with varying fuel/air mixtures and flow rates with equivalence ratios ranging from 0.2 to 1.0 and chemical energy inputs ranging from 2W to 16W. For this entire range self-sustained combustion was achieved in the device for temperatures up to an in excess of 300C, and catalytic ignition was possible at approximately room temperature.

K. Maruta and K. Takeda of AkitaPrefecturalUniversity along with J. Ahn, K. Borer, L. Sitzki, and P.D. Ronney of the University of Southern California and O. Deutschmann of the University of Heidelberg [5] analytically studied the extinction limits of a 1mm by 10mm cylindrical tube catalytic combustor burning methane. Computations were carried out with varied equivalence ratios, inlet velocities, and heat loss coefficients to determine extinction limits for the device. It was found that a minimum temperature of approximately 1300K was needed to sustain catalytic combustion. In an effort to reduce this temperature, computations were done in which the fuel-air mixtures were diluted with nitrogen to the extinction limit, holding the fuel-oxygen ratio fixed. The fuel concentration and temperature required to sustain catalytic combustion was nearly halved when this dilution occurred.

To test the results of the computational experiments, a “Swiss roll” combustor burning propane was tested under similar conditions to those used in the computations. This particular combustor was used with propane because methane required temperatures beyond the burner limits to sustain combustion. Use of a different combustor design and a different fuel also helped in determining whether or not the results of the computations were dependent on geometry and fuel. The tests showed similar results to those calculated for catalytic combustion, but did not show similar results for non-catalytic combustion. Both the computational and experimental results suggest that dilution of the fuel-air mixture with nitrogen or the products of the reaction is more effective at reducing the temperature required to sustain combustion than operation at lean equivalence ratios.

J.M. Hatfield and R.B. Peterson of OregonStateUniversity [6] tested for sustainable combustion conditions in a microcombustor burning propane utilizing platinum as a catalyst, heat recirculation from the exhaust gases to preheat the reactants and increase the adiabatic flame temperature, and quartz tubing to reduce the heat loss through the walls of the microcombustor. The experiments showed an increase in reaction temperature with increased fuel flow rate, appearing to approach a maximum temperature with increasing equivalence ratios at a slightly rich equivalence ratio. As conditions became more rich temperatures decreased, appearing to approach a rich flammability limit. As flow rates were increased, temperature oscillations increased, with oscillations of ±150°C at a fuel flow rate of 2sccm and an equivalence ratio of 1.3. Also contributing to system instability was lack of insulation around the quartz, allowing convective heat transfer to reduce the reaction temperature. Despite the instabilities, self-sustaining catalytic combustion did occur in the reactor at temperatures ranging from 400°C to 900°C.

Motivation and Proposed Design

J.R. Weiss [7] began work on developing a microscale catalytic combustor for use in a solid piston microengine at the PennsylvaniaStateUniversity in 2002. The microengine will use catalytic combustion on a platinum coated hastelloy tube to create thermal strain in the tube, which will generate electrical output by creating a force on a piezoelectric film. The catalytic combustion process is the focus of this paper. Previous microcombustor work has stopped short of fabrication and testing of a microcombustor. This paper describes the development and testing of a microcombustor test rig. This microcombustor consists of a platinum coated hastelloy tube with an outer diameter of 0.065in inside a hastelloy tube with an outer diameter of 0.25in and an inner diameter of 0.18in. Preheated air enters a cap at one end of the tubes, where it is mixed with hydrogen. The mixture then enters the combustion chamber, which houses the catalyst, and the exhaust gases are passed through a cap at the opposite end of the tubes. The test rig is encompassed by a ceramic insulator to reduce heat loss from the combustor. The data collected and knowledge gained from the experiments conducted on this test rig will be used to develop a sustainable catalytic microcombustor to power the single piston microengine.

REFERENCES

  1. Peirs, J., Reynaerts, D., and Verplaetsen, F., “Development of an Axial Microturbine for a Portable Gas Turbine Generator,” Journal of Micromechanics and Microengineering13, pp. S190-S195 (2003).
  1. Spadaccini, C.M., Mehra, A., Lee, J., Zhang, X., Lukachko, S., and Waitz, I.A., “High Power Density Silicon Combustion Systems for Micro Gas Turbine Engines,” Journal of Engineering for Gas Turbines and Power125, pp. 709-19 (2003).
  1. Peterson, R.B., and Vanderhoff, J.A., “A Catalytic Combustor for Microscale Applications,” Combustion Science and Technology Communications1, pp. 10-13 (2000).
  1. Vican, J., Gajdeczko, B.F., Dryer, F.L., Milius, D.L., Aksay, I.A., and Yetter, R.A., “Development of a Microreactor as a Thermal Source for Microelectrochemical Systems Power Generation,” Proceedings of the Combustion Institute29, pp. 909-16 (2002).
  1. Maruta, K., Takeda, K., Ahn, J., Borer, K., Sitzki, L., Ronney, P.D., and Deutschmann, O., “Extinction Limits of Catalytic Combustion in Microchannels,” Colloquium Topic.
  1. Hatfield, J.M., and Peterson, R.B., “A Catalytically Sustained Microcombustor Burning Propane,”ASME IMECE ASED Session on Miniature and Microscale Energy Systems (2001).
  1. Weiss, J.R., “A Microscale Catalytic Combustor for Use in a Solid Piston Microengine,” Undergraduate Honors Thesis, Department of Mechanical and Nuclear Engineering, The PennsylvaniaStateUniversity (2003).